X-ray film cassettes are used in a variety of medical and industrial applications to house an x-ray film sheet. The use of a cassette facilitates handling of the film to protect the film from exposure to light before and after exposure to x-rays. The cassette also holds the x-ray sheet flatly in place during the x-ray exposure.
X-ray cassettes are typically constructed of a front cover and an opposing back cover. Both covers are usually constructed of an x-ray transparent material such as aluminum and are pivotably connected to one another by a hinge. One cover is typically surrounded at least partially by a recessed channel which receives a rib extending from the other cover. When the front and back covers are pivoted on the hinge toward one another to close the cassettes the rib mates with the channel in the opposing cover. This forms a barrier which prevents outside light from entering the cassette. Examples of such cassettes are those disclosed in U.S. Pat. Nos. 3,001,071 and 3,157,787.
One problem associated with the cassettes described above is the possible retention of air pockets adjacent the film inside the cassette when the cassette is closed. These air pockets affect the transmission of x-rays through the cassette and produce artifacts which degrade the quality of the x-ray image and impair its readability.
To overcome this problem of air pockets, it is known to provide an x-ray film cassette with at least one resilient cover with a bowed shape which arches inwardly when the cassette is open and which flattens due to engagement with the interior of the cassette as the cassette is pressed closed. A latching mechanism holds the cassette closed maintaining the flattened shape of the cover. Upon releasing the latch, the cassette springs open slightly due to the cover reassuming its arched shape. The interior of the cassette generally includes a pair of image intensifying screens between which a sheet of unexposed film is initially loaded. As the covers are closed, the cover flattens to press the film firmly between the screens. Due to the arched shape of the cover, pressure is initially asserted only near the center line of the film. As the cassette closes further, the region of pressure spreads toward opposite sides of the film as the cover continues to flatten until the cassette is completely closed. The increasing pressure spreading outwardly in this manner expels any air pockets which might otherwise have become trapped against the surface of the film. An example of such a cassette is disclosed in U.S. Pat. No. 4,032,790.
Another problem in producing clear x-ray pictures is the presence of scattered x-rays. Scattered x-rays are produced when x-rays strike an object within the x-ray subject so as to cause them to reflect at an oblique angle to the object. Scattered x-rays usually occur at the boundaries of a structure within an x-rayed object and blur the image of edges of the structure. To mitigate the effects of scattered x-rays, it is well known to interpose an anti-scatter grid between the x-rayed object and the film.
Anti-scatter grids are usually constructed of lead strips separated by spacers of aluminum, plastic, bakelite or other x-ray transmissive material. The lead strips and aluminum strips are dimensioned and arranged so that scattered x-rays are absorbed by the lead strips while other x-rays are permitted to pass through the spacers to the film. Eliminating scattered x-rays in this manner produces a clearer x-ray image. U.S. Pat. No. 4,380,087 notes that it has been known to encase such a grid in a rigid sheath of aluminum or other material and to lay the grid exteriorly of the x-ray incident side of the cassette. The aforementioned patent also discloses forming a grid integrally with a flat cover of the cassette. Such an arrangement is advantageous in a number of respects. The grid is captured by the cover and does not require handling as a separate element. The cover also helps to protect the grid from mechanical damage. However, such a flat cover is incapable of operating to expel air pockets in the manner described above since it is not prestressed to arch when open and resiliently flatten upon closing the cassette.
In view of the foregoing, it is an object of this invention is to provide an x-ray film cassette which combines the advantages of a cassette having a captive grid for absorbing scattered x-rays with those of a cassette having a cover adapted to expel any air pockets adjacent the surface of a sheet of film housed by the cassette. It is a further object of the invention to provide a method of making such an x-ray cassette.